Background/Purpose: Syndecans (SDCs) are type-I transmembrane proteoglycans that prominently interact with heparan sulfate, extracellular matrix (ECM) components and glycoproteins. The present study was carried out to determine the role of syndecans in promoting tissue destruction mediated using human rheumatoid arthritis synovial fibroblasts (RASFs) and a rat adjuvant-induced arthritis (AIA) model of RA

Methods: Human RASFs and healthy (NL) SFs were isolated from de-identified RA and NL synovial tissues, respectively, under an IRB approved protocol. RASF and NLSF lysates were prepared to study the expression of SDC-1, SDC-2, SDC-3, and SDC-4 using qRT-PCR and Western immunoblotting. Knockdown of SDCs using siRNA approach was conducted to study its effect on IL-1β (10 ng/ml) or TNF-α (20 ng/ml) induced signaling pathways and downstream inflammatory mediators in RASFs. Ankle, heart, spleen and liver homogenates from naïve and AIA rats were analyzed for differences in the expression levels of SDCs. Serum levels of SDC-2 and SDC-4 were also analyzed in RA patients and in AIA rats. p<0.05 was considered significant.

Results: Our qRT-PCR results showed that the expression of SDC-2 (~320%) and SDC-4 (~80%) was significantly higher in RASFs when compared to NLSFs, which was also confirmed in protein levels using Western blotting method. No significant changes were observed in SDC-1 and SDC-3 expression in RASFs compared to NLSFs. Stimulation of RASFs with IL-1β or TNF-αinduced SDC-2 and SDC-4, but not SDC-1 or SDC-3, expression compared to the un-stimulated controls. Knockdown of SDC-2 and SDC-4 significantly inhibited IL-1β-induced matrix metalloproteinase-1 (MMP-1) and MMP-13 production, which could partly be due to the decrease in ERK and PKCδ activation in RASFs. In addition, we also observed a statistically significant decrease in IL-1β-induced IL-6 production with the knockdown of SDC-2 or SDC-4. Evaluation of the tissue homogenates from naïve and early (day 8) and established (day 18) AIA in rats for SDC expression showed a marked increase in SDC-2 and -4 expression in serum, ankles, liver, and spleen in AIA group at day 8 and at day 18 compared with naïve group. Treatment of methotrexate (MTX) significantly decreased both SDC-2 and -4 expression compared to the AIA alone group. However, human RA serum samples showed a consistent increase only in SDC-4 expression compared to the serum from healthy donors.

Conclusion: These findings suggest SDC-4 may be a potential therapeutic target in regulating the role of cytokines and MMPs in mediating tissue destruction in RA.